The present invention relates to cryogenic packages for devices that operate at very low temperatures, i.e., 150K or below. The invention is more concerned with a package which can maintain a low temperature device at an even cryogenic temperature, without subjecting it to thermally induced stresses.
Many present day semiconductor devices exist which must be kept at cryogenic temperatures, e.g. liquid nitrogen temperature or below, in order to operate. One example is a superconductor device which must be kept below a critical temperature T.sub.c. Another example is a high speed processor which achieves high carrier mobility only at very cold temperatures. Another example is a low-noise amplifier (LNA) which operates at cryogenic temperatures to reduce the effects of thermal noise. Many of these devices have an irregular shape, which can make conventional cooling difficult.
With present day technology, the device is housed in a Dewar and a cold finger extends into the Dewar. The cold finger typically contacts the device and removes the heat generated in the device. Most preferably, the heat generating part of the device is in thermal contact with the cold finger. The cold finger achieves its cryogenic capacity typically through a closed cycle mechanical refrigerator, or through an open cycle gas expansion, or using a liquid or solid cryogen. The refrigeration achieved at the end of the Cold finger is distributed through conduction, i.e., through a heat-conducting platform, to the device to be cooled. This works well only if the devices are extremely planar, and can withstand thermal stresses. If the device is non-planar in form, temperature distribution becomes uneven, and thermal gradients appear from one part of the device to another.
In cryogenically cooled devices of this type, it is required that temperature distribution be as even as possible, to avoid temperature gradients appearing along thermal conduction paths. Temperature gradients can induce stresses where the materials have variations in their coefficients of thermal expansion (CTE). CTE stress may also result from the platform on which the device is mounted or restrained. These stresses can degrade device performance, and can lead to catastrophic failure where materials are not well matched. Temperature gradients also degrade or induce varying performance in devices, requiring uniform temperatures throughout.
Immersion of the device into a liquid cryogen, e.g. liquid nitrogen, is sometimes used for cryogenic cooling of devices of irregular shape. However, immersion cooling is limited to the boiling temperature of the liquid. For nitrogen, this temperature is about 77K. It is not possible to cool a device in this fashion to a predetermined temperature between helium and nitrogen boiling temperatures. Also, because the cryogen is liquid in form, the system is quite orientation-sensitive and cannot be used in a mobile or space environment where the liquid would not remain in place.